The present invention relates to a process of reactivating an active cathode, especially an active cathode used with an ion exchange membrane type electrolyzer.
For brine electrolysis, a cathode has been used, which is coated on its surface with an electrode catalyst material and has a lowered hydrogen generation overvoltage. However, the electrode catalyst material coating is freed of the electrode catalyst material or the catalytic action of the electrode catalyst material drops under the influences of impurities, etc. in a cathode solution while an electrolyzer is operated over long years, resulting in a hydrogen generation potential rise.
So far, various processes have been used to recover such a lowered activity of the electrode catalyst material coated on the cathode. For instance,
(1) an electrode catalyst coating with a decreased activity is partly or wholly removed from the surface of a cathode attached to an electrolyzer body, and a fresh electrode catalyst is re-coated on the surface of a cathode substrate;
(2) a cathode substrate with a decreased cathode surface activity is removed from an electrolyzer, a fresh electrode catalyst material is re-coated on the surface of the cathode substrate, and the cathode substrate is attached to the electrolyzer; and
(3) a first porous cathode substrate is detachably provided thereon with a second cathode substrate smaller in wire diameter than the first cathode substrate by way of a flexible or other member, and a catalytically active substance is coated on the second cathode substrate, so that when the activity of the catalytically active substance drops, the second cathode substrate is reactivated upon being removed from the first cathode substrate.
However, a problem with process (1) is that when the cathode is reactivated in remote facilities while it remains attached to the electrolyzer, the electrolyzer must be shut down over an extended period because of the need of long transportation times, etc.
A problem with process (2) is that it is required to remove from the electrolyzer body the cathode substrate that is generally welded thereto. In other words, the removal of the cathode substrate from the electrolyzer body, the attachment to the electrolyzer body of the cathode substrate with a fresh cathodically active substance re-coated thereon, and other operations must be performed at exclusive facilities.
An electrolyzer to which process (3) can be applied is constructed from an electrically conductive, porous electrode or cathode substrate and an electrode made up of a fine substrate of small diameter, which is attached to the porous electrode substrate by way of a flexible or spongy member. In such an electrolyzer, electrolysis takes place while the electrode is in close contact with an ion exchange membrane, and electrically conductive connection is made between the electrode and the porous electrode substrate by bringing the surface of the ion exchange membrane in contact with the surface of the electrode with a given contact surface pressure. Thus, the ion exchange membrane should have a contact surface pressure enough to make such electrically conductive connection. For this reason the ion exchange membrane may possibly be injured by the fine electrode substrate, and gases produced during electrolysis stagnate between the electrode and the porous electrode substrate.